A novel method for the determination of bonded area of individual fiber-fiber bonds

A method is presented for determining the bonded area of single fiber-fiber bonds, the method is based on microtome serial sectioning and image analysis. The size and three-dimensional structure of the bonded area are assessed together with cross sectional fiber morphology. Additionally, holes and overlapping but unbonded fiber regions can be measured. 87 fiber-fiber bonds from an unbleached and unbeaten soft-wood kraft pulp were analyzed. The statistical evaluation of the results showed that basic geometry (fiber width and crossing angle) explain only 55% of bonded area. Incomplete bonding (holes and overlapping but unbonded edges) additionally account for 27% of the bonded area, while fiber morphology only plays a minor role. This is a contradiction to the conventional theory that fiber conformability controls the bonded area in paper. An explanation for this might be that one cannot rationalize the three-dimensional network of paper directly from two-dimensional fiber-fiber bonds.